21 is an odd composite number. It is composed of two distinct prime numbers multiplied together. It has a total of four divisors.
Prime factorization of 21:
3 × 7
3 × 7
See below for interesting mathematical facts about the number 21 from the Numbermatics database.
Names of 21
- Cardinal: 21 can be written as Twenty-one.
- Scientific notation: 2.1 × 101
Factors of 21
- Number of distinct prime factors ω(n): 2
- Total number of prime factors Ω(n): 2
- Sum of prime factors: 10
Divisors of 21
- Number of divisors d(n): 4
- Complete list of divisors:
- Sum of all divisors σ(n): 32
- Sum of proper divisors (its aliquot sum) s(n): 11
- 21 is a deficient number, because the sum of its proper divisors (11) is less than itself. Its deficiency is 10
Bases of 21
- Binary: 10101 2
- Hexadecimal: 0x15
Squares and roots of 21
- 21 squared (212) is 441
- 21 cubed (213) is 9261
square rootof 21 is 4.5825756949
cube rootof 21 is 2.7589241763
Scales and comparisonsHow big is 21?
To count from 1 to 21 would take you about ten seconds.
This is a very rough estimate, based on a speaking rate of half a second every third order of magnitude. If you speak quickly, you could probably say any randomly-chosen number between one and a thousand in around half a second. Very big numbers obviously take longer to say, so we add half a second for every extra x1000.
Note: we do not count involuntary pauses, bathroom breaks or the necessity of sleep in our calculation!
- A cube with a volume of 21 cubic inches would be around 0.2 feet tall.
Recreational maths with 21
- 21 backwards is 12
- 21 is a
- The number of decimal digits it has is: 2
- The sum of 21's digits is 3
- More coming soon!
The information we have on file for 21 includes mathematical data and numerical statistics calculated using standard algorithms and methods. We are adding more all the time. If there are any features you would like to see, please contact us. Information provided for educational use, intellectual curiosity and fun!